This was done using a mixture of magnetite, chalk, and home-brewed linseed oil as an underpainting. It's good for that purpose because it dries quickly and has a nice feel, somewhere between charcoal and sumi ink. I was trying to primarily catch the movement of the snow and letting the static structures fall into place as the picture developed. , and a video of me in the act
My paintbox sheltered by Artistic In Tent